Magnetic resonance imaging (MRI) apparatuses are diagnostic imaging apparatuses for medical use, which apply a radio frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, measure signals generated from the subject by nuclear magnetic resonance, and form an image from the signals. There are mainly two kinds of MRI apparatuses, i.e., tunnel type apparatuses utilizing a horizontal magnetic field and open type apparatuses utilizing a vertical magnetic field, those of the former type apply a static magnetic field in the direction parallel to the body axis of the subject, and those of the latter type apply a static magnetic field in the direction perpendicular to the body axis of the subject.
With the MRI apparatuses, images can be obtained for arbitrary imaging planes. The imaging planes include those of the three kinds of sections perpendicular to one another, i.e., axial sections dividing the body into a head side and a leg side, coronal sections dividing the body into a belly side and a dorsal side, and sagittal sections dividing the body into a right side and a left side, as well as oblique sections obliquely dividing the body at arbitrary angles.
In MRI apparatuses, in general, a slice gradient magnetic field for specifying an imaging plane, and an excitation pulse for exciting magnetization in the plane (radio frequency magnetic field pulse) are simultaneously applied, and magnetic resonance signals (echoes) generated at the time of convergence of the excited magnetization are obtained. In the above process, in order to impart positional information to the magnetization, a phase encoding gradient magnetic field and a read-out gradient magnetic field perpendicular to each other on the imaging plane are applied during the period from the excitation to acquisition of the signals. The measured echoes are arranged in the k-space having a kx horizontal axis and a ky vertical axis, and subjected to inverse Fourier transform to perform image reconstruction.
Each of the pixel values of the reconstructed image is a complex number consisting of an absolute value and a declination (phase). These absolute value and phase are determined depending on the static magnetic field intensity, direction of the static magnetic field (B0 direction), imaging parameters such as type of imaging sequence, pixel size, and repetition time, magnetization density in the subject, relaxation time (T1, T2), and so forth.
For usual diagnoses, gray-scale images of which pixel values are absolute values (absolute images) are used. The absolute images are advantageous for depiction of tissue structures, and include various kinds of images, such as proton (hydrogen nucleus) density-weighted images, T1-weighted images, T2-weighted images, diffusion-weighted image, and vascular images. On the other hand, gray-scale images of which pixel values are phase values (phase images) are images reflecting spatial distribution of magnetic field intensity. The phase images are frequently used for adjustment of measurement parameters or the like, but are not used for diagnosis so frequently.
However, the SWI (susceptibility-weighted imaging) method based on the fact that the phase images reflect difference of magnetic susceptibility between tissues has been developed in recent years, and actively utilized (refer to, for example, Patent document 1). The SWI method is a technique for image processing, and in this method, weighting is performed for an absolute image by using a phase image. In the images obtained through the image processing according to the SWI method, there are imaged veins, small hemorrhagic lesions, siderotic tissues, and so forth, containing a lot of paramagnetic substances and showing a higher magnetic susceptibility value compared with surrounding tissue.
In the SWI method, a phase mask image as a weighting image in which signal intensities of negative phase regions are reduced is formed first by using a phase image, and an absolute image is multiplied by this phase mask image to form an image in which the negative phase regions are emphasized in black (magnetic susceptibility-weighted image). Further, when the magnetic susceptibility-weighted images are displayed, the minimum intensity projection (minIP) processing is performed for a plurality of continuous magnetic susceptibility-weighted images. The minIP processing is one of the methods for projecting a plurality of images in one image. In the projected image, the minimum pixel value among the corresponding pixel values of the plurality of image to be projected is set as the pixel value for every pixel.
The contrast of the phase images reflects local change of magnetic field caused by the magnetic susceptibility difference between tissues and changes depending on the B0 direction. Therefore, the contrast of the magnetic susceptibility-weighted images generated by the SWI method, which uses a phase image for creation of the weighted images, also changes depending on the B0 direction. For example, it is known that the magnetic susceptibility-weighted images created by the SWI method suffer from a B0 direction dependency, for example, veins in an imaging slice substantially perpendicular to the B0 direction can be emphasized, whereas imaging ability for veins of the other imaging slices, especially an imaging slice parallel to the B0 direction, is reduced (refer to, for example, Patent document 2).